1. Field of the Invention
Embodiments of the subject matter disclosed herein generally relate to methods and devices that automatically set a speed of an expander, which receives a fluid flow output from another expander to be positively or negatively biased, in order to decrease a transition time through a speed range that is unsafe for the integrity of one of the expanders.
2. Description of Related Art
In gas and oil refrigeration systems, often two expanders are arranged in series, and are used to cool a refrigerant gas. This refrigerant gas is a cooling agent for liquefying the natural gas. FIG. 1 is a schematic diagram of a conventional two expander assembly 1. A gas flow output from a first expander 10 enters a second expander 20, the “first” and “second” labels being related to expanders' positions in a flow direction 30.
The first expander 10 typically receives gas having a high pressure at room temperature, and outputs gas having a low pressure and a low temperature. The second expander 20 receives the gas output from the first expander 10 and proceeds cooling the gas. The first expander 10 and the second expander 20, which expand the gas, have rotating impellers 22 and 24, respectively. During normal operation, when there are no concerns related to avoiding a speed range for one of the expanders, a regulator 40 sets a rotating speed of the impeller 24 of the second expander 20 to be the same as a current rotating speed of the impeller 22 of the first expander 10. The regulator 40 may receive information on the current speed of the first expander 10 from a speed sensor (Sv1) 50.
In the following description, the term “speed” includes “rotating speed,” and the term “speed of an expander” is used instead of repeatedly specifying “speed of an impeller of an expander.” The speeds of the expanders 10 and 20 are related to a gas flow passing therethrough, the speeds increasing when the gas flow increases.
As known in the art, for an expander, there is usually at least one undesirable operating speed. When the expander functions at the undesirable operating speed for an extended time, damage is more likely to occur than when operating at other operating speeds, for example, because excessive vibrations occur at the undesirable speed due to a resonance phenomenon. Therefore, operators try to avoid operating the expanders at the undesirable speed, by controlling the expanders such as to operate as short time as possible, in an undesirable range around the undesirable speed.
Conventionally, in order to avoid operating one of the first expander 10 or the second expander 20 in their respective undesirable range, the speed of the second expander 20 is manually set to deviate from the speed of the first expander 10. Setting the speed of the second expander 20 to be different from the speed of the first expander 10 has the effect of changing a distribution of the pressure drop across the expanders. Therefore, the speed of the first expander 10 is affected by the manner in which the speed of the second expander 20 is set. By controlling the set speed of the second expander 20, an operator may indirectly also control the speed of the first expander 10.
The manual operation of the system has the following disadvantages. Manually biasing the set speed of the second expander 20 is associated with high risk of accidentally operating one of the expanders inappropriately. In addition to biasing the speed of the second expander, the operator should control the system to comply with constraints related to a maximum allowed running time inside the undesirable speed range, a maximum allowed rate of a change of the set speed, and a maximum allowed speed difference between the expanders.
Another disadvantage is that, in case of a manual operation, the undesirable range is often defined to be broader than minimum necessary, thereby reducing a normal operating range for the expander.
Manually biasing the speed of the second expander 20 may also result in difficulties in operating the whole system in a controlled manner. For example, the rate of change of the set speed should be maintained smaller than a threshold value in order to allow the two-expander system to achieve equilibrium operating states, instead of operating in potentially harmful and hard to control transition states. When the speed is set manually, this rate of change of the speed may accidentally become too large.
Additionally, a manual operation aimed to decrease a time of operating an expander in an undesirable speed range may distract the operator from the overall monitoring of the system, which may result in a delayed response to unrelated abnormalities that may occur concurrently with the manual operation.
Accordingly, it would be desirable to provide systems and methods that avoid the afore-described problems and drawbacks.